1. Field of the Invention
The invention relates to a vehicle shock absorber comprising a damping medium-filled damping cylinder divided into two damping chambers, a compression chamber and a return chamber, by a main piston attached to a piston rod. The main piston moves axially in the damping cylinder in a compression and return movement. When the shock absorber reaches a certain compressed state, the movement is slowed by a function that increases the damping.
2. Description of the Related Art
This type of shock absorber is disclosed, for example, by EP0565832, GB998742 or the applicant's patent EP1006292B1, in which the compression movement is slowed by a second piston, herein referred to as the damping piston, which at a certain stroke length reaches a cup having a smaller diameter than the damping cylinder.
It has proved a problem, in a shock absorber with this type of progressive damping, to create a gentle slowing of the damping movement in the limit position. When the second piston reaches the cup of smaller diameter, the piston has a high velocity, which generates a large braking force around the point of maximum engagement. The counter-force then diminishes as the velocity decreases.
A shock absorber configured according to EP1006292, for example, has a second piston with a number of thin washers, a so-called shim stack, which opens under a certain force. The opening force is generated when the pressure in the cup acts on the opening area of the shim stack. The opening characteristic of a shim stack can be varied by adjusting the size, thickness, number and reciprocal arrangement of the washers (i.e., the shims). A relatively progressive damping characteristic is most often created, which creates a force that increases with velocity, which leads to large increases in the force per unit time when the second piston comes into engagement at a high velocity. This behavior can be a problem for a driver of the vehicle, because the slowing may be perceived as jerky. Rapid changes in the force between the wheel and the road surface risks leading to rapid changes in the distribution of the adhesion between the road surface and the different wheels. This makes handling of the vehicle more difficult, since it becomes less predictable. It is therefore desirable, instead of having a counter-force that increases with increasing velocity, to have a counteracting damping force that is as constant as possible throughout slowing of the movement.
EP0565832 also illustrates a third problem of the known technology, which is the problem of creating a function that will afford a fully controllable slowing of the damping movement without the second piston at the same time contributing to a geometrically over-defined system, which may become locked or require extremely precise tolerances. In EP0565832 this problem is resolved in that the second piston has an outside diameter that is significantly smaller than the inside diameter of the cup, and in that any movement in a radial direction is absorbed by a flexible washer. EP1006292 also resolves this problem in that the second piston has an outside diameter that is smaller than the inside diameter of the cup. In this case, however, the leakage flow of damping medium that passes on the outside of the piston is deducted from the total braking function, that is to say the braking characteristic is not solely determined by the opening characteristic of the shim stack. At low piston velocities the braking characteristic is in principle determined solely by the magnitude of the leakage flow.
In both cases this leads to a very progressive damping characteristic, which is to say the damping force increases sharply with the velocity, with all the implicit disadvantages described above.